In fifth-edition Dungeons & Dragons, “aberrations” are monsters that come from somewhere other than the material world that the player characters inhabit—from other planes or even other universes. We can still presume that they’ve evolved, although the conditions they’ve evolved in may be very different from our own, and that they still behave in ways that further their own survival, whatever powers or alien thought processes they may possess.
The nothic is categorized in the 5E Monster Manual as an aberration, although based on its flavor text, it seems like it belongs more in the category of “monstrosity,” which includes beings created by magic: nothics are described as onetime wizards whose avarice for secret knowledge led to their being cursed by the lich Vecna. This “origin story” seems to make them cousins to the undead as well. Since monstrosities and undead are not evolved creatures, it’s hard to say whether nothics should play by the usual rules of natural selection or not. I’m going to examine them as weird hybrids that follow those rules sometimes but are also subject to undead-esque compulsion.